Mechanical and rheological characterization of hydrogels and hydrogel precursors for cartilage tissue engineering and biofabrication

Motivation: Hydrogels have a wide range of possible applications in medical technology, for example in contact lenses, wound-healing dressings, drug delivery devices and in tissue regeneration. In particular, biomaterials for tissue regeneration require challenging engineering and control over material properties. The characteristics of hydrogels material properties make them excellent candidates for tissue engineering approaches. For this reason, a detailed understanding and characterization of hydrogel properties are required. A comprehensive characterization of the viscoelastic behaviour of hydrogels can result in a better understanding of cell-material interactions, while chondrocyte cell response and cartilage matrix formation are key requirements for successful cartilage regeneration therapies. This work, in particular, deals with the mechanical and rheological properties of hydrogels for cartilage tissue engineering and biofabrication. Materials and Methods: Three different scaffold materials, ADA-GEL (alginate-di-aldehyde and gelatine), alginate-gelatine and commercially available Chondrofillerliquid (Amedrix GmbH) were investigated to evaluate the hydrogels stiffness and viscoelastic relaxation behaviour. The mechanical tests were performed in a 37°C environment to simulate physiological conditions. All scaffolds underwent an uniaxial and unconfined compression of 15%. ADA-GEL was assessed in terms of rheological properties necessary for 3D printing of hydrogel precursor inks. Results and Discussion: The analysed materials showed different Young's moduli, ranging from 9 to 21 kPa. The relaxation time ?1/2 when the stress is relaxed to half of its initial value was measured. ?1/2  ranged from 4.5 to 143.5 seconds. ADA-GEL precursor showed strong shear thinning and temperature dependent properties, depicting the interval of printability. Conclusion: The results indicate that the different chemical compositions of the materials can significantly alter the detailed rheological properties, mechanical behaviour and stress relaxation, while the latter has been found to play a crucial role in cartilage tissue engineering.